Generally, in a facility for using a nuclear fuel material and an equipment for transporting the same such as various nuclear reactor facilities, reprocessing facilities and nuclear fuel material storing facilities, it is "security of subcriticality" that is basically required for the nuclear fuel material.
As a fissile material such as 235-U and 239-Pu is easily fissile by a thermal neutron (several eV), it is necessary to absorb the thermal neutron to reduce a thermal neutron flux, thereby enhancing the subcriticality. Thus, in the aforementioned facilities or equipments, metal materials having high thermal neutron absorptivity are widely used.
Especially, in recent years, high enrichment of the nuclear fuel accompanied by high burnup has been promoted in a commercial reactor. Further, in a research reactor, the nuclear fuel of much higher enrichment is used. Therefore, it is necessary to employ metal materials having higher thermal neutron absorptivity for a basket that is used as a fuel storing structure in a spent fuel transporting and storing vessel.
Further, as the basket is subject to high temperature (200.degree.-300.degree. C.) due to decay heat of the spent fuel, the metal materials requires increased high-temperature strength.
The conventional metal materials utilizes superior thermal neutron absorptivity owned by B. Examples of the metal materials may include Boral (trade name by Brooks & Perkins Co.), aluminium alloy bond-casting a mixed sintered material of B.sub.4 C and Cu, B-containing aluminium alloy, B-containing stainless steel and B-containing steel.
(1) Boral:
Boral is prepared by sintering a mixture of B.sub.4 C and Al, sandwiching the sintered material between Al plates, and rolling the whole. The content of B in the sintered material is high, but it is reduced because the sintered material is sandwiched between the Al plates. If B.sub.4 C is increased, workability is remarkably deteriorated. Therefore, the content of B.sub.4 C is about 3.5 wt.% at present.
(2) Aluminium alloy bond-casting a mixed sintered material of B.sub.4 C and Cu:
The content of B.sub.4 C is required to be increased for the purpose of enhancing the thermal neutron absorptivity. However, if the content of B.sub.4 C is increased, the sintered material is embrittled, and cannot be shaped. The content of B in the sintered material is about 28 wt.%, but the content is reduced because the sintered material is bond-casted in the aluminium alloy. Furthermore, when the sintered material of B.sub.4 C and Cu is bond-casted, there is generated gas from the sintered material. As a result, it is difficult to obtain a desired ingot.
(3) B-containing aluminium alloy:
Al-B alloy wrought material and casting containing 2-5 wt.% of B are used at present. Segregation of B is remarkable, and uniform composition is hard to obtain. Further, as a melting point is remarkably increased by adding B into Al, it is considered that the mass-produceable content of B is 5 wt.% or less, and it is hard to obtain an aluminium alloy containing B of more than 5 wt.%.
(4) B-containing stainless steel and B-containing steel:
These materials are a stainless steel containing B and a carbon steel containing B. As the content of B increases, workability is deteriorated, and hot forging or hot rolling is greatly difficult. Further, a mechanical property at room temperatures is deteriorated. Therefore, at present, the content of B is obliged to be limited to 2 wt.% or less for the carbon steel, and less than 2 wt.% for the stainless steel.
In this way, the Al alloy and the Fe alloy containing B as a thermal neutron absorbing material are practically used at present. However, in any of these materials, if the content of B is increased to enhance the thermal neutron absorptivity, material characteristics are deteriorated, and difficulty in manufacturing is increased. In contrast, the metal materials having good material characteristics contain little content of B to cause low thermal neutron absorptivity.